Reasonably new owner 1998 sedan. It idles a bit rough at stop. I was told this is common ( yes and I was born Yesterday) where should I look first? I added a cold air intake but it was doing this before the add on. Some days it runs like a scared rabbit and some days it is a dog... wonder if it is related to the idle. Thanks for your input and patience!

This is definitely not common, my 02/98 M3 idles smooth.
Did you measure for air leaks in intake system, temperature sensors for air intake and coolant?
Has the crankshaft position sensor already been replaced on your 1998 M3 under the recall against a new one with 12V instead of the old 5V version plus a new harness for that? If not, or not known, check with your dealer if there are any recalls open.